Theoretical evaluation of 15N isotopic methods for measuring symbiotic nitrogen fixation in the fied

International audience; Isotopic methods for the measurement of symbiotic N2 fixation by leguminous plants in the field rely on the use of differences in 15N enrichment between the N sources potentially available for leguminous crops, soil mineral N and atmospheric N2 . This methodology has been fully documented, especially concerning limitations due to non uniform and non constant distribution of 15N and to the use of a reference plant to measure it. Although all authors recognise the necessity of isotopic methods for giving yield independent and time-integrated estimates of symbiotic fixation, they also agree that these methods intrinsically remain imperfect. Our aim in this chapter is (i…